1. Field of the Invention
The present invention relates to a novel Mg based alloy and a novel Mg casting alloy capable of mass producing OA parts, car parts, electric appliance parts and so on through die casting, injection molding or the like, and relates to articles mold-cast using the alloy.
2. Description of the Prior Art
The casting Mg alloys practically used in present time are as follows:
(1) AZ, AM alloys (Mgxe2x80x94Alxe2x80x94(Zn)xe2x80x94Mn system, for example, ASTM: AZ91D);
(2) AS alloy (Mgxe2x80x94Alxe2x80x94Sixe2x80x94Mn system, for example, ASTM: AS41); and
(3) AE, QE, WE alloys (an alloy group containing one or more kinds of REM, Ag, Y).
The alloy (1) is most commonly used as the die casting and the injection molding Mg alloy, and particularly the AZ91D is good in die-castability and in corrosion resistance and widely applied to car parts and electric appliance parts. The alloys (2), (3) are alloys improving the mechanical properties such as the creep property and the high temperature strength. As the prior art in regard to these alloys, various kinds of alloys are disclosed in the following patent gazettes.
For example, Japanese Patent Application Laid-Open No.6-330216 discloses an Mg based alloy containing Ca, Si, Al, Zn and Mn, Japanese Patent Application Laid-Open No.9-104942 discloses an Mg based alloy containing 5 to 10 of Al, 0.2 to 1 of Si and 0.05 to 0.5 of Cu, and Japanese Patent Application Laid-Open No.10-147830 discloses an Mg based alloy containing 1 to 6 of Gd and 6 to 12 of Y.
With growing needs of thin thickness and high precision of parts in order to reduce in weight and size of potable devices, high fluidity alloys have been required. The alloy (1) of AZ91D described above is comparatively high in the fluidity, but the molding yield in injection molding is not always sufficiently high.
The alloy groups (2), (3) are prior to AZ91D in the mechanical properties such as creep property, strength at high temperature. However, because of the bad fluidity, the alloy groups (2), (3) are apt to cause casting cracks in the molding method of high speed cooling such as the injection molding method and are bad in castability.
The fluidity may be improved by raising the temperature of molten alloy. However, raising of the molten alloy temperature has problems in oxidation of the molten alloy and in shortening of durable lifetime of the production machines. Therefore, it is necessary to improve the fluidity by the other method.
It is known that the solidification structure of AZ91D becomes dendritic when it is cooled in a comparatively slow speed such as at ingot casting. As described above, the alloy is designed by placing special emphasis on the molten fluidity, and in regard to the properties after solidification, the alloy is designed so that the various kinds of properties such as the mechanical properties are optimized on the premise that the structure of AZ91D becomes dendritic.
However, in the cases of die casting and injection molding to which the alloy is widely applied, it is known that the structure after solidification becomes the cellular structure not the dendritic structure because the cooling rate is very fast. Therefore, it is required to change the designing method of the conventional alloying composition.
An object of the present invention is to provide a high strength Mg based alloy and a Mg based casting alloy having a good fluidity and a good mechanical property, and a cast article using the alloy.
As a result of various kinds of studies in order to solve the problems described above, it is found that the melting point of the alloy is lowered and the fluidity is improved by adding appropriate amounts of Al, Sn and Zn to a magnesium alloy, and the present invention is established.
The present invention is characterized by a high strength Mg based alloy, which contains 2 to 20% of Al by weight; 0.1 to 10% of Zn; 0.1 to 15% of Sn; and 0.05 to 1.5% of Mn, or preferably the remainder which is consisting essentially of Mg.
The present invention is characterized by a high strength Mg based alloy, which contains 2 to 20% of Al by weight; 0.1 to 10% of Zn; 0.1 to 15% of Sn; and 0.05 to 1.5% of Mn, and has crystal size of 10 to 300 xcexcm, or preferably the remainder which is consisting essentially of Mg.
The present invention is characterized by a high strength Mg based alloy, which contains 8 to 20% of Al by weight; 0.1 to 5% of Zn; 0.1 to 10% of Sn; and less than 1.5% of Mn, and has a tensile strength (x) at 20xc2x0 C. larger than 240 MPa; and an elongation ratio (y) larger than 0.5% and at the same time larger than a value calculated by y=xe2x88x920.295x+78, or preferably the remainder which is consisting essentially of Mg.
The present invention is characterized by a high strength Mg based alloy, which contains 12 to 15% of Al by weight; 0.1 to 5% of Zn; 1 to 10% of Sn; 0.1 to 0.5% of Mn, and the remainder contains Mg more than 75%, or preferably the remainder which is consisting essentially of Mg.
The present invention is characterized by a high strength Mg based alloy, which contains 12 to 15% of Al by weight; 0.1 to 5% of Zn; 1 to 10% of Sn; 0.1 to 0.5% of Mn; one kind or more than two kinds of elements selected from the group consisting of Ca, Si and rear-earth elements of which the total content is less than 5%; at least one kind of element selected from the group consisting of Sr and Sb of which the total content is less than 1%; or preferably the remainder which is consisting essentially of Mg.
The present invention is characterized by a Mg based casting alloy, which contains 2 to 20% of Al by weight; and 0.1 to 15% of Sn; or preferably the remainder which is consisting essentially of Mg.
The present invention is characterized by a Mg based casting alloy, which contains 2 to 20% of Al by weight; 0.1 to 10% of Sn; and less than 1.5% of Mn, or preferably the remainder which is consisting essentially of Mg.
The present invention is characterized by a Mg based casting alloy, which contains 10 to 15% of Al by weight; 0.5 to 3% of Zn; 1.5 to 4.5% of Sn; 0.05 to 0.5% of Mn, or the remainder which is consisting essentially of Mg.
The present invention is characterized by a Mg based casting alloy which is prepared by that the above-mentioned Mg based casting alloys are added with one kind or more than two kinds of elements selected from the group consisting of Ca, Si and rear-earth elements of which the total content is less than 5% by weight; and at least one kind of element selected from the group consisting of Sr and Sb of which the total content is less than 1%, or the remainder which is consisting essentially of Mg.
The present invention is characterized by a die cast article or injection molding article, which is casted using a molten metal of any one of the alloys described above.
The present invention is characterized by a thixotropic mold article, which is molded using a molten metal of a mixture of liquid phase and solid phase of any one of the alloys described above.
In detail, it is preferable that the magnesium based alloys described above are formed in desirable shapes through die casting by injection molding.
The magnesium alloys in accordance with the present invention are improved in the fluidity due to lowering of the melting point particularly by adding a small amount of Sn to the Mg based alloy containing Al, and accordingly members having less surface defects can be obtained. Further, since low temperature molding can be performed and accordingly the contraction at solidifying is small, members having a high dimensional accuracy can be obtained. Therefore, the molding yield can be largely improved.
Further, since the load to the machines, for example, the cylinder of an injection molding machine or the like is decreased, the durable lifetime of the heat resistant materials can be lengthened.
Furthermore, the magnesium alloys in accordance with the present invention are good in mechanical property and corrosion resistance because of the homogeneous and fine microstructure.
For the purpose of solid-solution hardening, precipitation hardening and improvement of fluidity, the element Al is added above 2%, preferably above 8%, particularly preferable above 12%. However, an excessive addition exceeding 20% of the element Al produces a large grain Mgxe2x80x94Al intermetallic compound to substantially decrease the elongation of the molded products. Further, in the casting method having a high cooling rate such as the die casting or the injection molding, the solidified structure becomes finer as the content of Al is increased, and the Mgxe2x80x94Al intermetallic compound does not grow large-sized, but is finely distributed in the crystal grain boundaries. This effect becomes obvious particularly when Sn is added together. In order to make the elongation above 3.5% and the tensile strength above 265 MPa, it is preferable to add 12 to 17% of Al.
Further, the element Al in the magnesium alloy in accordance with the present invention is solved in the xcex1-Mg phase, and reduce the melting point of the alloy. Further, the element Al is solid-solved in the xcex1-Mg phase and crystallizes the Mgxe2x80x94Al intermetallic compound, with the result that the strength at room temperature of the alloy is improved. Furthermore, the element Al suppresses oxidation of the molten alloy, and improves fluidity of the molten alloy. In order to attain these effects, the Al content is above 12%, and preferably above 15%.
The element Sn is solved in the xcex1-Mg phase, and reduce the melting point of the alloy with a small amount of nearly 0.1%, particularly more than 0.5%. Further, the element Sn is solved in the xcex1-Mg phase and crystallizes the Mgxe2x80x94Sn intermetallic compound, as a result the strength at room temperature is improved. Furthermore, the effect of Sn on lowering the melting point becomes obvious particularly when Al and Zn are added together, but the effect is almost saturated when the Sn content becomes 5%. Further, when the Sn content exceeds 15%, the elongation is largely decreased, and the density of the alloy becomes large, and lose the advantage of lightness of the magnesium alloy. Particularly, the Sn content needs to be lower than 10% in order to keep the elongation above 3.5%, and the Sn content needs to be preferably lower than 8% in order to keep the elongation above 4%. When the Sn content is 1 to 7%, it is possible to obtain an alloy having both of high strength and high elongation.
The element Zn is added above 0.1% in order to improve the strength at room temperature and the castability. However, when the Zn content exceeds 10%, casting cracks are apt to occur. It is preferable that the Zn content is within a range of 0.1 to 5%, preferably 1 to 5% in which the strength is high and the casting cracks do not occur.
The element Mn improve the corrosion resistance this is because Mn forms a intermetallic compound with Al, and fix Fe in the intermetallic compound, the element Fe being contained in the alloy as an impurity deteriorate the corrosion resistance. When the Mn content exceeds 1%, the Alxe2x80x94Mn group intermetallic compound excessively deposited and cause an evil effect on the mechanical property, the upper limit of Mn content is set to 1%. Particularly, for the corrosion resistance, Mn content is effective above 0.05%, and preferably 0.1 to 0.5%.
The alloy in accordance with the present invention further contains at least one element selected from the group consisting of Ca, Si and rear-earth elements, the content of the one kind or in total being less than 5%; and at least one element selected from the group consisting of Sr and Sb, the content of the one kind or in total being less than 1%. The elements Ca and Si and rear-earth elements are effective to lower the melting point because these elements form eutectic groups with Mg. However, since addition of these elements deteriorates the casting property, the upper limit of the content is 5%. Particularly, it is preferable that the content is above 0.1% and the upper limit is set to 3%.
The elements Sr and Sb make the metallic structure fine, and to improve the mechanical properties. The effect of elements Sr and Sb is increased when the element Si or Ca is added together. The effect of elements Sr and Sb is increased as the content is increased, but the effect is saturated when the content exceeds 1%. Therefore, the upper limit is set to 1%. Particularly, it is preferable that the content is above 0.03%, and the upper limit is set to 0.5%.
The Mg based alloy in accordance with the present invention is characterized that the surface is covered with an oxide film which contains Mg of 15 to 35% by atoms; preferably 20 to 30%, and Mo of 5 to 20%. The Mg based alloy in accordance with the present invention is characterized that the surface is covered with an oxide film which contains Mg of 15 to 35% by atoms; Mo of 5 to 20; and metallic Al of less than 30%, preferably 10 to 25%. The Mg based alloy in accordance with the present invention is characterized that the surface is covered with an oxide film which contains Mg of 15 to 35% by atoms; Mo of 5 to 20; oxide Al of less than 15%; and metallic Al of less than 15%, preferably 4 to 12%. The Mg based alloy in accordance with the present invention is characterized that the surface is covered with an inert oxide film of which a natural immersion electric potential 30 minutes after immersing into an aqueous solution of 0.01 mol Na2B4O7, pH 9.2, 25xc2x0 C. is higher than xe2x88x921500 mV, preferably higher than xe2x88x921400 mV. The Mg based alloy in accordance with the present invention is characterized that the surface is covered with an oxide film of which a natural immersion electric potential 15 minutes after immersing into an aqueous solution of 0.01 mol Na2SO4, 25xc2x0 C. is higher than xe2x88x921500 mV, preferably higher than xe2x88x921400 mV. Further, the Mg based alloy in accordance with the present invention is characterized that the surface is covered with the above-described oxide film or a specified oxide film, and a water repellent organic film containing fluoride is further coated on the oxide film.